Ventilating a shed

by Mark
(Kansas)

Ventilating a shed
by Mark
(Kansas)
John, I have a gambrel style 12’x16’ shed. The old siding has seen better days, so I’ve decided to replace the worn-out siding with a new board and batten siding. I’m also going to insulate and finish the inside with plywood, install a dog kennel, add some new base cabinets, workbench and add electrical.

My major concern is shed ventilation.

I have no idea what I need here, if anything. My future plans will be to add an electrical heater, so the dog and I can be comfortable all winter. We’re not going to condition the shed; we just need to have good ventilation all year round, so it’s not too hot in the summer or too cold in the winter.

I’m thinking of adding one Whirlybird vent on the roof through the attic into the main space.

Don’t know if this idea is good or bad.

Any suggestions?

Thanks

Mark

Answer

Hi Mark,

Thank you for your question.

Before I go into my opinions on this I should perhaps back up a bit and look at the reasons why anyone might want to ventilate their shed and the options that are available.

Why would I want to ventilate my shed?

The two reasons for ventilating a shed are to disperse excess heat in the summer and also to prevent the moist, damp, stale air in the winter months. Having proper ventilation is a great way to make it much more pleasant to spend time in your shed.

The main reason that a shed heats up in the summer is due to the sun shining directly onto the roof. Because the roof is pointing almost directly at the sun it absorbs significantly more energy than the walls. This build-up of heat can make it uncomfortably hot in the shed for humans or animals and in the extreme can warp timber or melt plastic components.

In the winter moist air is more of a problem. Stale damp uncirculated air allows fungal spores to grow causing potential problems in a storage shed such as mildew damage to household items. In extreme cases rotting of timbers exposed to long-term condensation can occur. Ensuring a supply of plenty of fresh air is a practical way to avoid this problem.

Dampness from the natural moisture content in the air is one element that can be magnified when humans or animals are occupying the shed. Due to the moisture added to the air from their breath and activity.

Funnily enough, improving insulation and the airtightness of your shed creates a greater need for ventilation.

What are your best shed ventilation options?

A shed can be ventilated by adding specifically designed vents to facilitate a healthy throughflow of air, in the roof, walls and eaves, gable and not forgetting the contribution to be made through simply opening a door or window.

A good venting system allows you to control and encourage natural ventilation.

Venting hot air to cool a building works on the following principle

The normal ventilation route is that warm air in the building rises, drawing in cool, clean air in through wall vents installed low down on the side of the shed.

As the hot air rises it then escapes through vents located high up in the roof or end walls of the shed.

For an attic area or two storey shed then using a soffit vent is the way to allow cooler air into the lower part of the upstairs room. A soffit vent can either be a slot running the full length of the eaves or individual vents at regular centres. This system still relies on hot air rising to the roof and escaping through a roof or ridge vent.

This method of air circulation works to lower the internal temperature in the shed by utilising temperature differences to ensure a healthy throughflow of air.

This type of ventilation is called passive ventilation as the air is drawn through the shed without any assistance.

Passive ventilation is what happens in most small sheds.

Active ventilation is where powered fans are used to assist the airflow.

What are the different types of vents?

Low level vents

Wall vents – These are located down at floor level to allow the influx of cooler, drier air. They often have a slider to control the amount of air flow. So that days when there is less need for throughflow of air it can be reduced.



High level vents

• Gable vents - are located high up on the gable wall of your shed so that warm air can escape. This location is good as having vents in opposing walls also encourages air flow through the building. Ensure that any vents you have are louvered. The louvers are there to allow air to flow out but to stop driven rain from entering the shed.
• Ridge vent – These are a proprietary item that allow air to escape from the ridge line of the shed. The exact design will depend on the type of ridge vent system that you choose.
• Caravan roof vent – These are normally a polycarbonate dome that can be opened to varying degrees to control the flow of air. Caravan roof vents are relatively small and the dome is designed to exclude rain when partially open
• Venting skylight – Venting skylights allow natural light into the shed and when opened allow the flow of air. They are more of a roof window and will provide a large vent area when open. Some even have a trickle vent that can be activated even when the window isn’t open.
• Cupola – Essentially a covered hole in the roof! A cupola has the advantage that it is located on the high point of the shed and is activated by wind from any direction. Make sure the sides have effective louvers. You can also fit a weather van to the top of the cupola as a decorative feature.
• Whirlybird (rotating cowl) – This is verging on active ventilation as the wind powers a turbine with a vertical axis to extract air from the shed roof.

One thing to note with all types of vent is that you have a potential route for unwanted insects to enter the building. I would recommend that you include a fly screen in any vent to keep out these little visitors!

What is the best passive ventilation strategy for a small shed?
If you have a small shed, a good ventilation strategy with a minimum of vents is to place them on opposite walls. Locate one vent low down, just above floor level on one side and have the other vent high up on the opposing wall.

This will facilitate a cross flow of air from the wind as well as taking advantage of heat rising within the shed as I described earlier.

What are the problems with passive shed ventilation?

Passive ventilation can be a bit hit and miss. It is generally dependent on wind direction, the strengths the wind blows, the local topography and distribution of trees and buildings can have a significant effect.

With more extreme climates and larger sheds, the air flow sometimes needs a little help in the form of active ventilation.

With active ventilation the natural flow of air is given some help

Active ventilation gives the air flow a hand by either drawing in cooler air or flushing out the hot air at the top of the building.

A healthy through flow of air can be achieved using a whirlybird (rotating cowl) type vent. This type of vent is situated on the shed roof and is powered by the wind. The faster the wind blows the more air is drawn through the vent.

Alternatively, a powered vent can be used, an electric fan will give a more controllable flow than a wind powered vent. A powered ventilation system will need its own electricity supply and management system. This could be from a mains supply or for ‘off-grid’ sheds a solar panel with back-up battery supply can provide the source of energy. (could be as simple as an on/off switch).

Good ventilation also helps to avoid moisture build up

I mentioned earlier about how heat rises. Well warm air also holds more moisture than cool air. So those vents that you installed high up in the shed also help to release dam air from your shed. A small fan can help in cooler weather when the temperature gradient isn’t sufficient.

A final note on active shed ventilation

The more airtight a shed is the more sophisticated the management system needs to be. In a large office type shed for example that is weather tight and occupied the whole year around then an air source heat pump might be used. This system would draw cool air into the building and then recover the heat from the warm stale air that it extracts so that minimum energy would be lost.

However, by the sounds of it, Mark the heat recovery system is a long way from what you are after.

Getting back to Marks question

I would say that a turbine generator, or 'whirlybird', type of vent would give this shed a good chance of venting the hot air out of the roof, with a bit more assistance than just relying totally on passive air flows. The whirlybird vent should be to remove the hot air from the top of the roof and so would not be required to be connected by ducting to the main space below. The hot air in the lower space would naturally rise up the building.

My concern here would be the lack of control in winter as you might then want to significantly reduce the airflow. So, an alternative could be gable vents with powered fans. These vents could then be significantly reduced in winter so that the heat loss is controlled.

The insulation to the walls will help to reduce the heat gain in the summer and will also reduce the heat loss in the winter months (You might like to have a quick look at this article on shed insulation).

How do you intend to insulate the roof/attic? It would be worth looking into insulating the ceiling to the main space depending upon exactly how you intend to use the building.

Hope these thoughts are of use and please let me know how you get on.

John